1
COPOLYMERIZATION
I. Morphology
A. Random ABBAABABABBBBA
1.
Each monomer has comparable reactivities with each other.
B. Alternating ABABABABABABABAB
1. Each monomer reacts exclusively with the other monomer
C. Block AAAAAAAAAABBBBBBBBBBAAAAAAAAA
1. Monomers may added in stages or
2. Pre-made oligomer blocks may react with each other.
D. Graft AAAAAAAAAAAAAAAAAA
B
B
B
B
B
B
1. Oligomer is attached as branch on main polymer backbone
E. Interspersed
1. Two monomers, each reacting exclusively with each other, form
interlocking crosslinked networks.
F. Blends
1. Some homopolymers can be blended to form a single-phase.
2. Many polymers are not miscible.
II. Kinetics
A. Consider two monomers that polymerize via free-radical polymerization.
(The scheme for ionic polymerization is very similar.)
k11
M1 i + M1 ⎯⎯
→ M1 i
k12
M1 i + M 2 ⎯⎯
→ M2 i
k 21
M 2 i + M1 ⎯⎯
→ M1 i
k 22
M 2 i + M 2 ⎯⎯
→ M2 i
B. Examine the rate of disappearance of each monomer by considering
propagation only.
d [ M1 ]
= k11 [ M1 i ][ M1 ] + k 21 [ M 2 i ][ M1 ]
dt
d [M2 ]
−
= k12 [ M1 i ][ M 2 ] + k 22 [ M 2 i ][ M 2 ]
dt
−
2
C. Assume steady-state of each radical.
⇒ k12 [ M1 i ][ M 2 ] = k 21 [ M 2 i][ M1 ]
⇒
[ M1 i] =
k 21 [ M 2 i ][ M1 ]
⇒
k12 [ M 2 ]
[ M 2 i] =
k12 [ M1 i ][ M 2 ]
k 21 [ M1 ]
D. Create ratio of disappearance rates
[ M1 ] ( k11 [ M1 i] + k 21 [ M 2 i])
d [ M 2 ] [ M 2 ] ( k12 [ M1 i ] + k 22 [ M 2 i ])
d [ M1 ]
=
E. Define reactivity ratios
r1 =
k11
k12
r2 =
k 22
k 21
1. r > 1, monomer has greater tendency to self-propagate
2. r < 1, greater tendency for copolymerization
F. Applying steady-state of radicals and definition of reactivity ratio, the ratio of
disappearance rates yield the copolymer equation.
⎛ k11 [ M1 i ]
⎞
+ 1⎟⎟
d [ M1 ]
⎝ k 21 [ M 2 i ] ⎠ = [ M1 ] ( r1 [ M1 ] + [ M 2 ])
=
d [M2 ]
⎛ k [ M i ] ⎞ [ M 2 ] ([ M1 ] + r2 [ M 2 ])
[ M 2 ] k12 [ M1 i] ⎜⎜1 + 22 2 ⎟⎟
k12 [ M1 i] ⎠
⎝
[ M1 ] k 21 [ M 2 i] ⎜⎜
G. Alternatively, the copolymer composition can be considered in terms of mole
fractions of monomer in a copolymer versus mole fraction monomer in feed.
f1 =
[ M1 ]
[ M1 ] + [ M 2 ]
f2 =
[M2 ] = 1 − f
1
[ M1 ] + [ M 2 ]
f1, f2 – mole fractions of monomer in feed
F1 =
d [ M1 ]
d [ M1 ] + d [ M 2 ]
F2 =
d [M2 ]
d [ M1 ] + d [ M 2 ]
F1, F2 – mole fractions of monomer in polymer
F1 =
r1f12 + f1f 2
r1f12 + 2f1f 2 + r2 f 22
= 1 − F1
3
H. Effect of reactivity ratios on copolymer composition
1. Case I: r1 = r2 = 1
a. No preference for monomer 1 or 2
b. Truly random copolymer
c. Example: ethylene : vinyl acetate r1 = 0.97 : r2 = 1.02
2. Case II: r1 = r2 = 0
a. Monomer 1 reacts exclusively with monomer 2 and vice versa
b. Alternating copolymer
c. Example: styrene : maleic anhydride r1 = 0.041 : r2 = 0.01
3. Case III: r1, r2 < 1
a. Copolymer has a tendency toward a pattern (like alternation)
i. r1 = 0.5 ⇒ M1 reacts twice as fast with M2 as with M1.
b. Composition of polymer is close to “azeotropic” concentration
i. azeotrope – composition of copolymer is equal to concentration of
feed
d [ M1 ] [ M1 ]
=
d [M2 ] [M2 ]
ii. Compare to azeotrope in liquid-vapor phase diagram where
composition of vapor is equal to composition of liquid.
c. Example: styrene : methyl methacrylate
r1 = 0.52 : r2 = 0.46
4. Case IV: r1 > 1, r2 < 1
a. Both M1 and M2 prefer to react with M1
b. Composition of copolymer changes significantly during copolymerization
(heavy in M1 initiallly, heavy in M2 at the end.
c. Example: styrene : vinyl chloride
r1 = 17 : r2 = 0.02
5. Case V: r1 ⋅r2 = 1
a. An “ideal polymerization”
b. Relative rates of monomer consumption independent of identity of
radical
c. Result is a random copolymer
d. Example: methyl methacrylate: vinyl chloride r1 = 10 : r2 = 0.1
4
Composition of copolymer
with various reactivity ratios
Case I
F1
(copolymer)
Case IV
Case II
Case III
f1 (feed)
I. Estimation of reactivity ratio
1. Reactivity ratios must be measured for each pair of monomers.
(Obviously inconvenient)
2. Estimation can be done with the Alfrey-Price “Q-e” scheme.
a. “Q” is a measure of reactivity via resonance stabilization.
b. “e” is a measure of polarity of monomer via effect of functional groups
on vinyl group.
3. The “Q-e” equations
2
k11 = p1Q1e − e1
k12 = p1Q 2e− e1e2
k 21 = p 2Q1e− e1e2
k 22 = p 2 Q2 e− e2
k
Q
⇒ r1 = 11 = 1 e− e1 ( e1 −e2 )
k12 Q 2
k
Q
⇒ r2 = 22 = 2 e − e2 ( e2 −e1 )
k 21 Q1
2
⇒ r1r2 = e
−( e1 − e2 )
2
c. If the polarities of the monomers are comparable, ideal polymerization
is expected.
4. More sophisticated “Q-e” schemes include factors for steric hindrance.
5
III. Properties and uses of copolymers
A. Styrene-butadiene (SBR, Buna S, Buna rubber)
1. Properties
a. Elastomeric
b. Abrasion resistance
c. Aging stability
d. Resistance to nonpolar solvents
2. Uses
a. Tires
b. Shoe heels and soles
c. Gaskets
B. Styrene –acrylonitrile (SAN)
1. Properties
a. Transparent
b. Heat resistant
i. Tg(PS) = 95 °C
Tg(SAN) = 195 °C
ii. Hydrogen bonding from acrylonitrile increases Tg.
c. Moderate tensile strength
d. Low impact resistance, brittle
e. Copolymer strengthens when crazed.
2. Uses
a. Food containers
b. Optical disks
c. Packaging
6
C. Acrylonitrile – Butadiene – Styrene (ABS)
1. Composition
a. Acrylonitrile – 15% to 35%
b. Butadiene – 5% to 30%
c. Styrene – 40% to 60%
d. Technically, ABS is a terpolymer.
2. Properties
a. Acrylonitrile adds toughness through hydrogen bonding.
b. Butadiene adds elasticity
c. Impact resistant
d. Heat resistant
e. Solvent resistant
i. Susceptible to H2SO4, HNO3, aromatic hydrocarbons, esters,
ketones, CCl4, glacial acetic acid.
3. Uses
a. Auto parts
b. Athletic helmets
c. Golf clubhead
d. Legos
e. Clarinets, recorders
7
D. Ethylene – vinyl acetate (EVA)
1. Properties
a. Impact resistance
b. Low temperature toughness
c. Transparent
d. Resistance to UV radiation
e. Vinyl acetate improves wettability of polymer
f. Elastomeric
g. Can be processed like a thermoplastic
2. Uses
a. Clothing foams
i. Boots
ii. Crocs
b. Substitute for cork
c. Time – release drug delivery
i. Drug is dispersed in polymer matrix.
ii. As water attacks polymer, matrix with drug slowly dissolves and
disperses.
iii. Copolymer is not metabolized.
E. Ethylene – tetrafluoroethylene (EFTE)
1. Properties
a. High tensile strength
b. Puncture resistant
c. Low coefficient of friction (“Low-stick”)
2. Uses
a. Protective gloves
b. Electrical insulation
c. Windows (Beijing water cube – 2008 Olympics)
8
F. Polyurethane – polyethylene glycol
1. Properties
a. Elastomeric (elongation 500%)
b. Low density
c. Abrasion resistant
d. Poor strength
e. Smooth, comfortable fabric
2. Uses
a. Spandex (fiber from DuPont)
b. Lycra (fabric from DuPont)
c. Athletic garments
G. Stryene – maleic anhydride
1. Properties
a. Transparent
b. High heat resistance
c. Low thermal expansivity
d. Anhydride groups make the copolymer soluble in alkaline solutions
e. Impact resistance
2. Uses
a. Optical fiber
b. Engineering plastic
c. Blended with ABS or PVC to increase heat stability
d. Coatings and adhesives
e. Compatibilizer (aids blending of ABS and nylon)
9